The invention relates generally to circuits for controlling current through a solenoid, motor or other load. The invention relates more specifically to a circuit for monitoring the load current to detect overcurrent and undercurrent conditions.
The following patent applications filed herewith have a common Detailed Description:
U.S. Ser. No. 07/782,218 filed on Oct. 24, 1991 by D. J. Ashley, M. K. Demoor and P. W. Graf, entitled "Temperature Compensated Circuit For Controlling Load Current" PA1 U.S. Ser. No. 07/782,833 filed on Oct. 24, 1991 by D. J. Ashley, entitled "Temperature Monitoring Pilot Transistor"
Solenoids and motors are used for various purposes, and may require carefully controlled load current at one or more levels. For example, some solenoids are used to drive print hammers in impact printers and require two controlled levels of drive current, an initial "activation" current and a subsequent hold current. The initial "activation" current is relatively large to overcome the inertia and static friction of the moving parts coupled to the solenoid, and the subsequent "hold" current is relatively low to limit the contact force or holding force of these moving parts.
U.S. Pat. No. 4,764,840 discloses a control circuit for a solenoid which circuit provides two levels of load current. A resistor is located in series with the solenoid, and the voltage across the resistor is supplied to the positive input of one comparator and the negative input of another comparator. The other inputs to the comparators are provided by a three resistor voltage divider which divides a reference voltage. Thus, the two comparators provide a window to control the voltage which drives the load current. While the reference voltage which drives the voltage divider is fixed, a one-shot injects extra current into the voltage divider during the activation current phase to raise the window for controlling the activation current.
In precision applications, it is vital to carefully control the drive current at each level particularly in view of temperature effects. Typically, there is a load transistor which conducts the load current, and the series on-resistance of the load transistor increases as the load transistor conducts because the conducted current heats the load transistor. Consequently, the changing on-resistance of the load transistor will effect the drive current. There are also other factors which affect the load current. As a result, some form of feedback has been utilized to continuously control the source voltage to compensate for such changes in the series on-resistance of the load transistor and the other factors. For example, a small resistor has been placed in series with the coil, and the voltage across the resistor used to measure the drive current. This measurement in turn is used to control the drive current. This technique has the disadvantages of power dissipation and imprecision due to the variation in resistance of the series resistor with temperature. A more recent (prior art) technique utilizes a "drain pilot" transistor which is a scaled version of the load transistor. For example, the load transistor is made of thousands of identical transistors in parallel and the pilot transistor has the same structure as the individual transistors of the load transistor and the size of hundreds of the individual transistors in parallel. The drain pilot transistor and the load transistor are both integrated into the same "chip" and are located adjacent to each other, but the drain pilot transistor does not pass any of the load current. Nevertheless, as the load transistor heats-up due to the load current, the pilot transistor also heats-up and the on-resistance of each changes proportionally. A constant current source feeds the pilot transistor and therefore, develops a voltage which is proportional to the ideal load current. The voltage across the pilot transistor is then compared to a voltage sensed across the load transistor. If the second voltage is greater than the reference voltage then the load is disconnected from the power source for a predetermined period. During this period, the load current will drop according to an RL time constant of the load circuit such that the sensed voltage falls below the reference voltage. Then, the power source is re-applied to the load to cause the load current to rise, and the cycle is repeated. Thus, the load current is controlled. While such control is accurate enough for many applications, the average load current can vary despite the accuracy of the reference voltage. This is because the amount of decay of the load current when the load is disconnected from the power supply depends on the resistance of the series load circuit and this resistance can neither be designed with precision nor kept constant with changes in temperature.
In precision applications, it is also desirable to monitor the drive currents at each level to determine if the drive current rises above or falls below an acceptable range at each level. Such a condition is called an overcurrent or undercurrent condition, respectively, and indicates that the controller for the drive current is malfunctioning and some corrective action should be taken.
Previously, it was known to establish two fixed reference voltage levels which bracket the acceptable voltage range at each level used to drive the solenoid. These two fixed reference voltages were compared to the corresponding load circuit voltages to detect overcurrent and undercurrent conditions at each level. In this prior art technique, the fixed voltage levels were not temperature compensated. If such a technique were used in conjunction with a temperature compensated load transistor, then the "window" between the two fixed voltage levels would have to be wide to accommodate the variations in the voltage levels which are sensed from the load circuit. In such a case, the overcurrent and undercurrent detector would not be sensitive enough for some precision applications.
Accordingly, a general object of the present invention is to provide a temperature compensated overcurrent and undercurrent detector.
A more specific object of the present invention is to provide a detector of the foregoing type which can be used with a drive circuit which utilizes a pilot transistor to provide temperature compensation for the voltage which drives the load current.
Another specific object of the present invention is to provide a detector of the foregoing type which can provide the detection window for two levels of drive current with a minimum amount of circuitry.
Still another object of the present invention is to provide a detector of the foregoing types which enables the windows to be moved by changing external parts to the integrated circuit and which requires a minimum number of changed parts and I/O pins to interconnect to the external parts.